In the field of liquid crystal display, an active layer of a thin film transistor is usually made of a silicon material with excellent stability performance and processing performance. The silicon material mainly includes an amorphous silicon material and a polycrystalline silicon material, and carrier mobility of the amorphous silicon material is very low. Although carrier mobility of the polycrystalline silicon material is high, the devices made of polycrystalline silicon materials have poor uniformity, low yield, and high unit price. Thus in recent years, the technology of using a transparent oxide semiconductor film in a channel forming region to form a thin film transistor (TFT) and applying the TFT in an electronic device or an optical device has received wide attention. A field effect transistor made of an amorphous In—Ga—Zn—O system material (a-IGZO) consisting of indium, gallium, zinc and oxygen has attracted most attention because of its high carrier mobility and large switching ratio. However, a-IGZO has an amorphous structure, and its performance is very unstable, so it is very important to improve the stability of an oxide semiconductor device. There have been proposed many methods to improve the stability of oxide semiconductor, and for example application of an upper and lower dual-gate structure is an effective method.
The oxide semiconductor device having a dual-gate structure can be manufactured by adding a metal layer as a top-gate electrode at the top of the TFT region of a conventional back channel etching (BCE) structure or an etching stop layer (ESL) structure. One patterning process for the metal layer and a deposition process for an insulating layer for protecting the metal are added to form the structure. The steps of forming the TFT of a dual-gate structure are increased, and the technology of forming the TFT of a dual-gate structure is complex.